diff --git a/magnum_cluster_api/json_patches.py b/magnum_cluster_api/json_patches.py
index 28fb4084..f8759c08 100644
--- a/magnum_cluster_api/json_patches.py
+++ b/magnum_cluster_api/json_patches.py
@@ -142,7 +142,7 @@ def definitions(self) -> list[dict]:
             ]
 
         if len(self.worker_disks) > 0:
-            definitions + [
+            definitions += [
                 {
                     "selector": {
                         "apiVersion": objects.OpenStackMachineTemplate.version,